Hydrolysis of cellulose esters



Patented Jan. 11, 1938 a UNITED STATES HYDROL YSIS F OELLULOSE ESTERS Carl J. Malm, Rochester, N. Y., assignor, by

mes'ne assignments, to Eastman Kodak C Ompans, Jersey City, N. J

Jersey a cor oration ofNew No Drawing. Application December 4, 1935, Serial No. 52,876

4 Claims.

The present invention relates to a method of hydrolysis in the preparation of lower fatty acid esters of cellulose in which the anhydride present in the spent esterification bath is hydrolized'with 5 aqueous organic acid before the addition of sulfuric acid which acts as the hydrolyzing catalyst. In the preparation of lower fatty acid esters of cellulose, such as cellulose acetate, the fully esterified cellulose isusually hydrolyzed by adding to its solution in the spent esterification mixture, a water-fatty acid mixture, with or without mineral acid, and allowing it to stand for a time. If no mineral acid is added with the water mixture, a long time of hydrolysis is required to obtain the desired product, especially if small amounts of catalyst havebeen used in the esterification. This results in tying up equipment and chemicals for a long period of time, thus increasing the cost of the final product. If, however, mineral acid is supplied by the water mixture, the time of hydrolysis is less than where no mineral acid is added but the product in thatcase has a lowered viscosity and melting point.

This is probably due to the degrading action of the catalyst at higher temperatures. Local overheating occurs when the water is added due to the reaction between the water and anhydride, which reaction is almost instantaneous in the presence of the comparatively large quantity so of catalyst.

One object of my invention is to provide a method for hydrolyzing lower fatty acid esters of cellulose and especially cellulose acetate in which the time'of hydrolysis is less than that 35 required when no mineral acid is added so that equipment and chemicals are not tied up for a comparatively long period of time. Another object of my invention is to provide a method of hydrolyzing lower fatty acid esters of cellulose 40 in which the viscosity and melting point of the ester is not lowered during the process. Another object of my invention is to provide a process for hydrolyzing lower fatty acid esters of cellulose in which the catalyst employed will not have a cor- 45 rosive action upon acetylation vessels, such as stainless steel or the like. Stainless steel isused to a considerable extent at the present time for the interior of acetylation apparatus such use being disclosed and claimed in my U. S. Patent I have found that cellulose esters may be hydrolyzed by myprocess in which additional mineral acid is added and yet at the same time the viscosity and the melting point of the ester are 55 not decreased in such a process. I have found a two-step method of hydrolyzing a cellulose ester which is in solution in its spent esterification bath in which first the anhydride present therein is hydrolyzed by adding only dilute lower fatty acid to the dope or solution and when the anhydride '5 has thus been converted into acid, a mixture of an aqueous solution of lower fatty acid with sulfuric acid is then added and the hydrolysis is carried out. I have found that by this means the degradation of the cellulose ester, which is evi- 10 denced by lowered viscosity and melting point, is avoided.

My invention is applicable to the hydrolysis of lower fatty acid esters in general, especially cellulose acetate, but including other lower fatty acid 15 esters of cellulose, such as cellulose propionate, cellulose butyrate, cellulose acetate-propionate, and cellulose acetate-butyrate. The hydrolysis of mixed fatty acid esters of cellulose is described in application Serial No. 551,546 of myself and 2 C. L. Fletcher, filed July 17, 1931.

As pointed out above, my hydrolysis method comprises two steps, namely, (1) the addition of sufiicient aqueous lower fatty acid to the spent esterification mixture to convert the anhydride 25 which might be present therein to the corresponding acid, after which the mixture is cooled down to the hydrolysis temperature, and 2) the addition of sulfuric acid with the remaining part of the aqueous lower fatty acid. It is not detrimental to my process if the amount of aqueous acid which is added to convert the anhydride to acid isin excess of that necessary for that purpose. However, in practice it is desirablethat not too great a proportion of this aqueous acid be added at that time, as it is desirable that the sulfuric. acid which is added in the second step be somewhat diluted which dilution is'readilvaccomplished with the remainder of the aqueous fatty acid to be added. 40

The hydrolysis of the ester is usually carried out in a bath containing up to about 10% of Water. However, if desired, the aggregate content of the water in the bath may be increased in. the manner described in the Malm and Fletcher application, Serial No. 651,138, filed January 11, 1933. Instead of the sulfuric acid being added all at once, it might be added a portion at a time. The time of hydrolysis however is shorter when the entire amount of mineral acid is added with the second portion of dilute fatty acid.

The following example illustrates my invention: 500 pounds of cotton linters were pretreated wlth a mixture of 3200 pounds of acetic acid and 8 pounds of sulfuric acid for two hours at 90 F. At the end of this time, the mixture was cooled to 65 F. and 1400 pounds of 85% acetic anhydride was added. The temperature was allowed to rise slowly to 100 F. where it was kept until a dope free from grain and fiber was obtained. 330 pounds of aqueous acetic acid was then added. The acid was added over a period of about a half an hour, during which time the temperature of the dope rose to 110-120 F. When the temperature ceased to rise after the addition of the acid the mass was cooled down to a temperature of 100 F. whereupon a mixture of 350 pounds of aqueous acetic acid (50%) and 16 pounds of concentrated sulfuric acid was slowly added. The mass was then allowed to stand at a temperature of about 100 F. for approximately 70 hours, after which it was precipitated, washed, and dried in the usual manner. The cellulose acetate resulting had an acetyl content of 38.5%.

, This product was compared with the products prepared by the ordinary method of hydrolysis and was found to have properties as good as those of an acetate in which no mineral acid of any kind was added in the hydrolysis step. For purposes of comparison, three cases of hydrolysis namely, (1) where the sulfuric acid is added with the aqueous acid in a one step addition (2) hydrolysis by addition of aqueous acid without any sulfuric acid, and (3) with the sulfuric acid added in accordance with the present invention were observed. In each case the hydrolysis of the acetate was carried out to an acetyl content of 38.5%. The results were as follows:

C 9 Time of Acetone Melting Oharring as hydrolysis viscosity point point Hrs. Secs. 0. C. 1 70 230 280 2 225 85 245 300 3 245 300 perature, then adding thereto further aqueous lower fatty acid in an amount sufiicient to induce hydrolysis, an amount of sulphuric acid sufiicient to accelerate the hydrolysis, and. maintaining the mixture at a hydrolyzing temperature until the desired product is obtained.

2. In a process of making a lower fatty acid ester of cellulose by procedure including esterifying cellulose with a bath containing acetic anhydride and a catalyst followed by a hydrolysis treatment of the esterification mixture to obtain an ester of acetone-solubility, the steps which comprise adding to the finished esterification mixture a quantity of aqueous acetic acid sufficient, but not greatly in excess of that necessary, to convert any anhydride present to acid, cooling the mixture to the hydrolysis temperature, then adding thereto further aqueous acetic acid in an amount sufficient to induce hydrolysis and sufficient sulphuric acid to accelerate the hydrolysis, and maintaining the mixture at about the hydrolyzing temperature until the desired product is obtained.

3. In a process of making cellulose acetate by esterification of cellulose with a bath containing acetic anhydride and catalyst, followed by hydrolysis of the esterification mixture to obtain cellulose acetate of acetone-solubility, the steps which comprise adding a quantity of aqueous acetic acid to the finished esterification mixture in an amount suflicient,-but not greatly in excess of that necessary to convert any anhydride present to the corresponding acid, cooling the mixture to a hydrolysis temperature, then adding to the cooled mixture a further quantity of aqueous acetic acid sufiicient to induce hydrolysis together with suificient sulphuric acid to accelerate the hydrolysis, and maintaining the mixture at a temperature which will hydrolyze the cellulose acetate to acetone-solubility.

4. In a process of making a lower fatty acid ester of cellulose by procedure including esterifying cellulose with a bath containing acetic anhydride and a catalyst, followed by a hydrolysis treatment of the mixture to obtain an ester of acetone-solubility, the steps which comprise treating the finished esterification mixture with aqueous acetic acid in an amount approximately sufficient to convert any anhydride present to the corresponding acid, cooling the mixture, then adding thereto further aqueous acetic acid in an amount sufiicient to induce hydrolysis together with several pounds of sulphuric acid to accelerate the hydrolysis, and maintaining the mix- I ture at a hydrolyzing temperature until the desired product is obtained.

CARL J. MALM. 

